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Business acquisitions - Summary of Consideration Transferred (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 01, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Business Acquisition [Line Items]        
Purchase price   $ 0 $ 1,042 $ 48,058
Mt. Holly Smelter [Member]        
Business Acquisition [Line Items]        
Purchase price $ 67,500      
Contingent consideration 13,780      
Economic, working capital and other closing adjustments (13,513)      
Total consideration $ 67,767